1. Field of the Invention
The present invention relates to a magnetic tape and a method of fabricating the same.
2. Description of Related Art
Conventionally, as magnetic recording tapes such as an audio tape, a video tape and so forth, a coating-type magnetic recording medium have widely been used, wherein a magnetic layer of which being formed by coating and drying a magnetic coating material, having a powdery magnetic material such as oxide magnetic powder, alloyed magnetic powder and so forth dispersed in an organic binder such as vinyl chloride/vinyl acetate-base copolymer, polyester resin, urethane resin, polyurethane resin and so forth, on a non-magnetic support.
On the other hand, for the purpose of application in data storage with an increasing demand on high-density magnetic recording, a magnetic recording medium of so-called, ferromagnetic metal film type has been proposed and has attracts public attention, wherein a magnetic layer of which being formed by directly depositing a ferromagnetic metal material such as Co—Ni-base alloy, Co—Cr-base alloy, Co—O or the like on a non-magnetic support such as polyester film, polyamide film, polyimide film or the like by a vacuum thin film forming process such as vacuum evaporation process, sputtering process, ion plating or the like, or by plating.
The above-described magnetic recording medium having a ferromagnetic metal thin film as the magnetic layer is excellent in coercive force, square ratio, and electro-magnetic conversion characteristics in the short-wavelength region, and is advantageous in many aspects, such as having an extremely small recording demagnetization and thickness loss during reproduction because the magnetic layer can be made extremely thin, and such as being successful in raising packing density of the magnetic material because there is no need of mixing a non-magnetic binder in the magnetic layer.
For the purpose of improving the electro-magnetic conversion characteristics and of obtaining a larger output of this sort of magnetic recording medium, there is also proposed an oblique evaporation by which the magnetic layer is obliquely deposited in the formation of the magnetic layer of the magnetic recording medium, and this has already been put into practical use as a magnetic tape for commercial video (8-mm, Hi-8 system, DV system) or professional-use video (DVCAM).
A magnetic tape, which is the above-described magnetic recording medium, is configured so that a magnetic layer typically composed of a ferromagnetic metal thin film formed by the oblique evaporation, and a protective layer for raising travel durability, composed of a carbon film called a diamond-like carbon (DLC) or a hydrogen-containing carbon film, and so forth are sequentially formed on a long non-magnetic support, and so that a lubricant layer, if necessary, is formed on the protective layer using a predetermined lubricant, and a back-coat layer is formed on the surface of the non-magnetic support opposite to that having the magnetic layer formed thereon.
Sputtering process and plasma CVD (chemical vapor deposition) process are techniques widely used for forming the protective layer, wherein the plasma CVD process has a larger opportunity of use in view of running durability and productivity.
In the formation of the protective layer applied with the plasma CVD process, a source gas is introduced into a vacuum chamber, an electrode is disposed so as to oppose with the magnetic layer deposited on the non-magnetic support, a plasma is excited by applying a high voltage between the electrode and magnetic layer to thereby decompose the source gas by the plasma, and to allow it to deposit as a DLC film on the magnetic layer.
Patent Document 1 describes a method of controlling film quality of thus-formed protective layer on the basis of a peak intensity (G) appeared at around 1,500 cm−1 and a peak intensity (D) appeared at around 1,300 cm−1 observed in Raman spectrometry, and a desirable range of D/G ratio.
In the above-described formation of the protective layer by the plasma CVD process, the magnetic layer deposited on the non-magnetic support is used as an electrode, wherein any changes in the film composition and thickness of the magnetic layer result in fluctuation in the voltage for exciting the plasma. Because properties of the carbon protective layer largely vary depending on the excitation voltage, it is difficult to obtain the same characteristics of the protective layer over the magnetic layers having different configurations. From another viewpoint of production, any compositional variation of the magnetic layer results in variation of the protective layer, and seriously degrades the productivity.
There is also a tendency towards a thinner thickness of the magnetic layer in association with increase in the recording density, and this tends to raise sheet resistance of the magnetic layer. The increase in the resistance of the magnetic layer makes it more difficult to apply a high voltage between the metal evaporated tape and the electrode.
The above-described event becomes distinct in particular for a thickness of the magnetic layer of 100 nm or less, and this substantially makes the film formation unavailable. FIG. 7 shows a graph plotting a thickness tmag of the magnetic layer on the abscissa, and plotting a ratio (D/G) of the peak intensity (G) appeared at around 1,500 cm−1 and the peak intensity (D) appeared at around 1,300 cm−1 observed in Raman spectrometry on the ordinate. The value D/G indicating the thickness of the protective layer varies with the thickness tmag of the magnetic layer, and a variable range for D/G can be altered by varying voltage applied to the magnetic layer and the electrode (V1, V2, for example), wherein the thickness tmag of the magnetic layer in a small region falls in a film unformable region Rimp where voltage application is impossible, which typically corresponds to a region of the thickness of the magnetic layer of 100 nm or less.
Besides this, a method of forming a DLC film is also described typically in Patent Document 2.
[Patent Document 1]
Japanese Patent Application Publication No. 2000-207735.
[Patent Document 2]
Published Japanese Translations of PCT International Publication for Patent Applications No. 2002-541604.